1. Field of the Invention
The present invention relates to a multi-frequency signal receiver, and more particularly to a receiver including a digital filter for detecting digitalized multi-frequency signals, such as dialing signals of push-phone type telephone sets or multi-frequency station selection signals.
2. Description of the Prior Art
Recently, in the art of a telephone switching system, the utility of a time division PCM switching system using a pulse code modulation (PCM) for a speech signal has been studied. In such a time division PCM switching system, a dialing signal of a push-phone (hereinafter referred to as PB signal) is also digitalized and coupled to a PB receiver via a time division speech path switching network. The PB signal as defined herein is a signal sent out to the switching system as a selection signal from a push-phone type telephone set, the signal frequencies of which comprise a group of lower frequency components including 697 Hz, 770 Hz, 852 Hz and 941 Hz and a group of higher frequency components including 1209 Hz, 1336 Hz and 1477 Hz. The PB signal consists of a combination of two frequency components, one selected from the lower frequency component group and the other selected from the higher frequency component group, to represent a dial number. For example, the combination of 697 Hz and 1209 Hz corresponds to dial number "1."
It has been proposed to receive such a digitalized multi-frequency signal by means of a digital filter. (For example, refer to the article "An Approach to The Implementation of Digital Filters" by L. B. Jackson, J. E. Kaiser, and H. S. McDonald, IEEE Trans. on Audio and Electroacoustics, Vol. AU-16, No. 3, 1968, pp 413-421).
The PB signal receiver using the digital filter has an advantage of being capable of multi-processing, that is, capable of processing a plurality of subscriber lines by a single receiver (or digital filter) because the processing speed of the digital filter is so high that it permits the reception of the PB signal from other subscriber line when the PB signal from a specified subscriber line is absent. However, in the prior art PB signal receiver using the digital filter, the digital filter has been used to discriminate the frequency components included in the PB signal as well as to determine the beginning, the duration and the end of the PB signal. Accordingly, so long as the PB signal in question lasts, the digital filter must be kept connected to the subscriber line of that PB signal. Thus, the prior art system has a drawback that the function of the digital filter is utilized only for the discrimination and the digital filter is wastefully kept connected during the duration of the PB signal after the completion of the discrimination.